In recent years, as a power source for an electrical device, there has been used a battery pack including secondary battery cells such as lithium-ion secondary battery cells.
The secondary battery cell included in the battery pack deteriorates depending on time elapsed from manufacturing, a cycle count representing the number of times that charging/discharging has been carried out, or the like, and the progress of the deterioration is accompanied by a reduction in the full-charge capacity of the secondary battery cell. In addition, when the deterioration of the secondary battery cell progresses to a prescribed level or more, not only the full-charge capacity may decline but also bulging or liquid leakage may occur in the secondary battery cell, thus exerting adverse effects on the electrical device including the battery pack. The full-charge capacity means a maximum power amount storable in the secondary battery cell.
For example, when bulging occurs in the secondary battery cell, the bulged secondary battery cell may press against the structural member (e.g., case) of the electrical device causing deformation of the device. When leakage of liquid occurs in the secondary battery cell which causes leaked electrolyte to stick to electronic components in the electrical device, the conductive electrolyte may migrate thereby causing deterioration of the electronic component.
Therefore, in some battery packs, protection circuits for suspending the use of the secondary battery cells before the secondary battery cells exert any adverse effect on the electrical device are provided.
The protection circuit of the battery pack measures the full-charge capacity or the direct-current resistance of the secondary battery cell, and predicts the deterioration state (State Of Health: SOH) of the secondary battery cell based on the measured value. The protection circuit then halts, based on the deterioration state, use of the secondary battery cell before the secondary battery cell can exert any adverse effect on the electrical device.